3.2.1 Change a Function Definition

If you want to change the code in multiply-by-seven, just rewrite
it. To install the new version in place of the old one, evaluate the
function definition again. This is how you modify code in Emacs. It is
very simple.

As an example, you can change the multiply-by-seven function to
add the number to itself seven times instead of multiplying the number
by seven. It produces the same answer, but by a different path. At
the same time, we will add a comment to the code; a comment is text
that the Lisp interpreter ignores, but that a human reader may find
useful or enlightening. The comment is that this is the “second
version”.

(defun multiply-by-seven (number) ; Second version.
"Multiply NUMBER by seven."
(+ number number number number number number number))

The comment follows a semicolon, ‘;’. In Lisp, everything on a
line that follows a semicolon is a comment. The end of the line is the
end of the comment. To stretch a comment over two or more lines, begin
each line with a semicolon.